What is Muscle Strength Testing
Muscle strength testing is an important component of medical examination and muscle evaluation.
Muscle strength is defined as the ability of a muscle to tighten (contract) and produce maximum force in a single effort.
Muscel strength depends on many factors
- Muscle cross-sectional area
- Musculotendinous stiffness
- Motor unit recruitment and synchronization
- Neuromuscular inhibition
In contrast, muscle endurance is how long a muscle can carry repeated contractions against resistance.
An assessment of muscle strength is can reveal information about neurologic issues.
Other names for muscle strength testing is
- Motor testing
- Muscle strength grading
- Muscle power grading
- Muscle testing
Need for Muscle Strength Testing
Muscle strength testing is done whenever there is a need for assessing muscle power.
Many conditions can affect muscle strength. These are
- Chronic illness-causing generalized muscle weakness
- Wasting due to disuse of the limb
- Neural diseases like
- Motor neuron disease
- Multiple sclerosis
- Brain injury
- Spinal injury
Muscle strength is also tested For assessing the effect of specific diseases like polio
- During rehabilitation and functional training for monitoring
- Replacement surgery
Other situations where muscle strength testing is done are
- Gait problems
- Assessment of fall risk
A physician will conduct muscle strength testing as a part of the medical examination. He would grossly look at the major muscle groups to ascertain if there is nervous system involvement or not.
In nerve palsies, the stress is upon specific muscle strength testing so as to diagnose the severity of palsy and evaluation of the recovery.
How is Muscle Strength Testing Done?
Manual muscle testing is the most common way to conduct muscle strength testing.
Manual muscle power testing and grading is a frequent and important part of neurological examination. The neurological examination consists of sensory and motor examination. Muscle power grading is part of the motor system examination.
Before muscle power grading is done, limbs or trunks are examined for the tone of the muscles, and the bulk of the muscles.
The limbs are inspected for their resting posture, deformity or clawing, and whether they are symmetrical.
Is there any wasting or hypertrophy?
Any involuntary movements, jerks, etc should be noted.
Muscle fasciculation is a sign of the lower motor neuron disease process. Muscle fasciculations are subcutaneous twitches over a muscle belly at rest.
Muscle tone is checked by moving the joints passively. A loss of tone suggests lower motor neuron lesions and hypertonia suggests upper motor neuron lesions.
Manual Muscle Testing
In this, depending on the target muscles to be tested movement is conducted against varying levels of resistance, and the strength is graded. For example, no flicker movement will be grade zero and the ability to make the movement against maximum resistance is grade 5.
This maximum resistance would vary according to the region or muscle tested. For example, maximum resistance for hip muscles would be far greater than for finger muscles.
As the muscles are tested, they are graded in power according to the scale below. The scale is known as the Oxford scale.
Individual muscles can be tested as in hand or feet. But in large joints like the hip or knee joint, groups of muscles like flexors, extensors or abductors are tested.
The patient is asked to contract the muscle group being tested and then the examiner resists the movement of that muscle group. Muscle power grading is done as per the following chart.
Here are the grades of muscle power as examined clinically
Grade 0– Complete paralysis, no contraction is seen or palpated
Grade 1 -a flicker of contraction present. Not able to produce joint motion with gravity eliminated
Grade 2– active movement with gravity eliminated. Able to move joint when gravity has been eliminated.
Grade 3-Active movement against gravity but not against resistance
Grade 4-Active movement against gravity and moderate resistance described as poor, fair, and moderate strength.
Grade 5– Normal power where the patient can move the joint against maximum resistance applied by the examiner
Sometimes. + or – are used to indicate the examiners’ observation that the power of the muscle is not up to the next grade but does not fit the lower grade too.
For example, a grade of 4+ indicates that a muscle has a strength somewhere between 4 and 5.
A 4- grade means that a muscle was near 4 but not that confidently.
Muscle power grading is useful in localizing the lesion and to differentiate the groups of muscles affected so that treatment can be affected.
False results can occur in
- Very weak muscles
- Joint contractures
- Exhaustion from repeated testing
This method of muscle strength grading is not entirely objective and subjective errors can creep in especially in very extreme grades. For example, it may be difficult to differentiate between grades 0 and 1 or 4 and 5.
Oxford scale has the following issues
- Grading is not linear
- The difference in power grade 4 and 5 are more than that between 0 and 1
- Patient cooperation is important
- Can vary with the examiner
It includes testing muscle power with a hand-held device called a dynamometer is positioned against the muscle.
The hand-held dynamometer is placed at precise locations on a patient’s limb by the examiner. The patient then exerts pressure against it for several seconds. A reading in pounds or kilograms is then displayed and measures the force generated by a particular muscle or a group of the muscles.
The reading is then compared to the reference values for age and sex.
The dynamometer is widely used to monitor the rehabilitation progress in physiotherapy.
Functional testing requires the patient to perform various maneuvers to note the ease and deficiencies of the function.
Functional testing is thought to provide a better relationship between strength and disability.
For example, pushing with the arms to get out of a chair may indicate quadriceps weakness.
Functional testing may be misleading about muscle power when the disability is due to something else. The pain in the region or joint stiffness may cause a decrease in function and it does not necessarily the muscle strength loss.
- Merck Manual How to assess muscle strength. from:https://www.merckmanuals.com/professional/neurologic-disorders/neurologic-examination/how-to-assess-muscle-strength (last accessed 08.5.2022)